Liposomes are closed vesicles composed of one or more lipid membranes mainly consisting of phospholipids and are used as models of biological membranes in physicochemical studies thereof. Furthermore, they can entrap various substances in the inside aqueous phase or in the membranes and therefore they have become a target of intensive investigations of their use as one of drug delivery systems containing various drugs entrapped therein. As regard the use thereof as such delivery system, there may be mentioned, among others, the report of B. E. Ryman et al. [Ann. N. Y. Acad. Sci., 308, 281 (1978)], the report of G. Gregoridias ("Liposome Technology", CRC Press Inc.) and the report of J. N. Weinstein [Science, 204, 188 (1979)]. In particular, they are effective in reducing the toxicity of highly toxic drugs, stabilizing drugs otherwise unstable in vivo and achieving sustained release of drugs in vivo. It is further known that they can be used as means of directing drugs to particular cells selectively by utilizing their property of fusing with cells or being taken up by cells. In a typical example of their application, liposomes are combined with, for example, enzymes (superoxide dismutase etc.), drugs (in particular antitumor agents, adriamycin), chelating agents, hormones (steroid compounds), radionuclides, interferons, interleukins, antigens and antibodies. It is also known that the duration of the efficacy of a drug showing a brief blood half-life, such as insulin, can be prolonged by entrapping it in liposomes. A therapy (missile therapy) has been developed as well which uses anticancer agent-containing liposomes combined with tumor surface-specific antigens bound thereto for selectively attacking cancer cells.
However, in their use in vivo, liposomes administered to the living body are captured by reticuloendothelial system tissues in a short time. This capture is a result of liposomes, which are macromolecules, being recognized as a foreign matter. Such characteristic is a great drawback when it is intended to direct liposomes to tissues other than RES tissues or to increase the duration of the efficacy of a drug in the blood circulation.
For removing such drawback, liposomes containing sialic acids in their lipid bilayer have been proposed (U.S. Pat. No.4,501,728). However, these liposomes cannot avoid capture by the RES to a sufficient extent, either, so that they cannot produce a satisfactory drug efficacy prolonging effect.